[1. Technical Field of the Invention]
The present invention relates to a segment-core type stator for radially-gapped inner-rotor type rotary electric machines and a method for manufacturing the stator.
[2. Related Art]
Recently, it is essential for the automotive industry to give serious consideration to the environment and power saving. Under such circumstances, EVs (electric vehicles) and HVs (hybrid vehicles) have been researched and put to practical use.
In vehicles, such as EVs and HVs, it is important to reduce vehicle weight that has strong positive correlation with fuel consumption. In this regard, there has been a strong demand for enhancing output per unit weight (kW/kgw) of a high-current motor (e.g., drive motor) loaded on such a heavy vehicle. It has been known that, in a stator incorporated in such a motor, the increase in a space factor of slots, i.e. the cross-sectional area of the coil conductor per unit cross-sectional area of slots, in the stator, has a strong positive correlation with the output per unit weight of the motor (kW/kgw).
One known method for winding a coil about a stator involves use of a coil winder. In the case of a conventional winding method, i.e. in the case of winding a fine round wire about one slat, using such a coil winder, it is necessary to ensure a space in the slot, to which the coil winder is inserted. In addition, the cross section of the coil conductor has a circular shape. For these reasons, the space factor of slots (hereinafter referred to “slot space factor”) of a stator about which a coil is wound has been as small as about 40%.
It should be appreciated that, throughout the specification, when a term “cross section” or “cross-sectional area” is used for a winding or a wire, the term refers to a cross section or a cross-sectional area perpendicular to the longitudinal direction.
If a flat wire having a large cross-sectional area can be used as a stator coil, the slot space factor may be remarkably improved. However, such a large-size flat wire cannot be wound about a stator with the use of a coil winder, unlike the case of winding a normal fine round wire. On the contrary, bending, per se, has been difficult in winding such a large-size flat wire. Therefore, when a stator is manufactured using a large-size flat conductor wire, the method may preferably include a step of bending the large-size flat conductor wire for preparation of a stator coil in advance, followed by a step of inserting portions of the conductor wire of the coil to be accommodated in slots (herein after referred to as “slot-accommodated portions”) into the respective slots of the stator core, for completion of a stator.
Stator coils are known to have concentrated winding or distributed winding. Comparing with the latter, the former is known to have an advantage, for example, of reducing torque ripple. A stator coil having a distributed winding of a large-size flat conductor wire will have the cage shape (hereinafter referred to as a “cage stator coil” or a “cage coil”). The outer diameter of such a cage stator coil will be approximately equal to that of the bottom surface of the slots of the stator core. For this reason, the stator coil cannot be easily inserted into a cylindrical stator core. Specifically, a cage stator coil prepared using a large-size flat square conductor wire of large cross-sectional area will have a very large rigidity. Therefore, it will be very difficult to position the slot-accommodated portions, as they are, radially inside the stator core having a relatively small inner diameter.
An approach known for avoiding the difficulty mentioned above in mounting a cage stator coil in the slots of a stator core, may be to use a segment core structure. Specifically, in this approach, a stator is divided into a plurality of segment cores. Then, the slot-accommodated portions of the cage stator coil are inserted into the slots of each segment core. After that, the segment cores are mechanically connected to complete a stator. Owing to the combination of such a segment-core type stator with a cage stator coil, an insulation-coated flat conductor wire having a large cross-sectional area can be used as a coil conductor to realize a radially-gaped inner-rotor type rotary electric machine having a good slot space factor.
Examples of such a segment-core type stator are disclosed, for example, in Japanese Patent Application Laid-Open Nos. 2002-141230 and 2003-235187.
In the segment-core type stator as mentioned above, the larger number of divisions of the stator core may more facilitate the assemblage of the stator core with the cage stator coil. However, the larger the number of divisions is, the smaller each of the segment cores may become. This, in turn, may complicate the process of mechanically connecting a number of segment cores, each having slots into which the slot-accommodated portions have been inserted.
Stator cores, meanwhile, are required to be ensured with good mechanical rigidity against magnetic vibration and external forces. However, the large number of segment cores with a large number of mechanical connecting portions may unavoidably deteriorate the mechanical rigidity of the stator core. In addition, the large number of segment cores may necessitate the increase of the magnetic resistance of the stator core, leading to deterioration in the output and the torque.
On the other hand, a segment-core type stator having a less number of divisions, such as two, may make it difficult for a cage stator coil with a relatively large diameter, to be inserted into the slots in the segment cores.